Technical Field of the Invention
This invention generally relates to the field of hydroprocessing catalysts for treatment of hydrocarbons. In particular, the present invention is directed to a process for preparing a catalyst useful for the hydrodesulfurization of gasoline feedstock with minimal loss of octane rating.
Description of the Prior Art
In the petroleum industry, it is common for gas oils, particularly middle distillate petroleum fuels, to contain sulfur species. Engines utilizing petroleum based fuels that include sulfur produce emissions of nitrogen oxide, sulfur oxide and particulate matter. Government regulations have become more stringent in recent years with respect to allowable levels of the potentially harmful emissions.
Many countries around the world currently limit allowable sulfur content in gasoline fuels to less than 50 ppm, and in some cases as low as 20 ppm. As environmental concerns grow, allowable sulfur content in gasoline fuels may soon be limited to 10 ppm or less. Thus, catalysts and processes for the production of gasoline fuels having a sulfur content of 10 ppm or less are needed.
Various methods have been proposed to reduce sulfur levels in gas oils. However, there are disadvantages associated with previously proposed methods. For example, hydrodesulfurization of fuel in catalytic reactors has been proposed, however the process frequently requires two or more reactors operating in series under severe reaction conditions; i.e., low flow rates and high temperatures, pressures and hydrogen consumption conditions. The severe reaction conditions are necessary to overcome strong inhibition by refractory sulfur and nitrogen compounds against hydrodesulfurization. Therefore, strict conditions are also imposed on apparatus design, thereby typically incurring high construction costs.
Alternatively, various organic and inorganic adsorbents have been proposed to effectuate adsorptive removal of sulfur compounds. Examples of previously proposed adsorbents include silica, alumina, zeolite, activated carbon, activated carbon-based fiber materials and spent hydrodesulfurization catalyst. However, the volumetric adsorption capacity for these adsorbents was often too low, and breakthrough of sulfur compounds into the fuel product was often too rapid. Also, inorganic adsorbents typically require high temperature treatment for regeneration, which is not practical for stable and continuous operation, and the adsorption regeneration cycle can be too frequent, which makes efficient operation difficult. Further, these adsorbents often can be expensive and susceptible to attrition. Fine particles produced due to attrition between adsorbent particles can cause plugging and high pressure drop, each of which can shorten the run length of an adsorption process.
Catalytic desulfurization is one method for removal of sulfur of hydrocarbons. Generally, catalytic desulfurization takes place at elevated temperature and pressure in the presence of hydrogen. At the elevated temperatures and pressures, catalytic desulfurization can result in the hydrogenation of other compounds, such as for example, olefin compounds, which may be present in the petroleum fraction which is being desulfurized. Hydrogenation of olefin products is undesirable as the olefins play an important role providing higher octane ratings (RON) of the feedstock. Thus, unintentional hydrogenation of olefin compounds during desulfurization may result in a decreased overall octane rating for the feedstock. If there is significant loss of octane rating during the hydrodesulfurization of the hydrocarbon stream, because of saturation of olefin compounds, the octane loss must be compensated for by blending substantial amounts of reformate, isomerate and alkylate into the gasoline fuel. The blending of additional compounds to increase the octane rating is typically expensive and thus detrimental to the overall economy of the refining process.
Additionally, catalytic hydrodesulfurization can result in the formation of hydrogen sulfide as a byproduct. Hydrogen sulfide produced in this manner can recombine with species present in the hydrocarbon feed, and create additional or other sulfur containing species. Olefins are one exemplary species prone to recombination with hydrogen sulfide to generate organic sulfides and thiols. This reformation to produce organic sulfides and thiols can limit the total attainable sulfur content which may be achieved by conventional catalytic desulfurization.
Alumina is a common support material used for catalyst compositions, but has several disadvantages in the desulfurization of petroleum distillates. Alumina, which is acidic, may not be well suited for the preparation of desulfurization catalysts with high loading of active catalytic species (i.e., greater than 10 weight %) for catalytically cracked gasoline. Acidic sites present on the alumina support facilitate the saturation of olefins, which in turn results in the loss of octane rating of gasoline. Additionally, recombination of the olefin with hydrogen sulfide, an inevitable result of hydrodesulfurization, produces organic sulfur compounds. Furthermore, basic species present in the feedstock, such as many nitrogen containing compounds, can bind to acidic sites on the surface of the alumina and the catalyst, thereby limiting the number of surface sites which are available for sulfur compounds for desulfurization. Furthermore, basic species present in the feedstock, such as many nitrogen containing compounds, can bind to acidic sites on the surface of the alumina and the catalyst, thereby limiting the number of surface sites which are available for sulfur compounds for desulfurization. At the same time, nitrogen containing compounds having aromatic rings are easily transformed into coke precursors, resulting in rapid coking of the catalyst. Additionally, high dispersion of the metal is difficult to enhance with an alumina support due to the strong polarity and the limited surface area of the alumina. Exemplary commercially available hydrotreating catalysts employing an alumina support include, but are not limited to, CoMo/Al2O3, NiMo/Al2O3, CoMoP/Al2O3, NiMoP/Al2O3, CoMoB/Al2O3, NiMoB/Al2O3, CoMoPB/Al2O3, NiMoPB/Al2O3, NiCoMo/Al2O3, NiCoMoP/Al2O3, NiCoMOB/Al2O3, and NiCoMoPB/Al2O3, (wherein Co is the element cobalt, Ni is nickel, Mo is molybdenum, P is phosphorous, B is boron, Al is aluminum and O is oxygen).
In addition, prior art methods suffer in that the preparation of desulfurization catalysts having high metal loading with high dispersion is generally difficult. For example, many prior art catalysts are prepared by a conventional impregnation method wherein the catalysts are prepared by mixing the support materials with a solution that includes metal compounds, followed by filtration, drying, calcination and activation. However, catalyst particles prepared by this method are generally limited in the amount of metal which can be loaded to the support material with high dispersion, which generally does not exceed approximately 25% by weight of the metal oxide to the support material. Attempts to achieve higher loading of the metal to support materials having a relatively high surface area, such as silicon dioxide, typically result in the formation of aggregates of metallic compounds on the surface of the support. Activated carbon has much higher surface area and weaker polarity than conventional catalyst supports, such as for example, alumina and silica. This provides improved performance in the desulfurization of catalytically cracked gasoline because both olefin saturation and recombination of hydrogen sulfide with the olefin are suppressed over activated carbon support. However, weaker polarity and a relatively high hydrophobicity make activated carbon difficult to load large amount of active metallic species, such as molybdenum oxide.
Thus, catalyst compositions and methods for preparing catalysts useful for the removal of sulfur species from petroleum based products are needed. Specifically, methods for the production of the catalyst compositions which include support materials having high surface area and high catalyst loading with high dispersion for the desulfurization of petroleum products are desired.